The present invention relates to a method for manufacturing a semiconductor thin film capacitor. More particularly, the present invention is directed to a method for manufacturing a thin film capacitor having a dielectric film formed of tantalum oxide.
The operational characteristics of thin film capacitors become increasingly important as the operation frequency of the various circuits in which these capacitors are included increases. Examples of such circuits include dynamic random access memories, in which the thin-film capacitor is employed as a storage cell; filters, in which the thin-film capacitor forms part of an RC network; and multi-chip modules, in which the thin-film capacitor is employed as a decoupling capacitor.
Operational characteristics that are desirable for a thin-film capacitor include high-capacitance density, low current leakage and a high breakdown voltage. In addition, it is desirable that the capacitors be compatible with subsequent steps during manufacturing of the circuit.
Due to its excellent dielectric properties, extensive efforts have been made to make capacitors using Tantalum Pentoxide (Ta2O5) films deposited by reactive sputtering, chemical vapor deposition (CVD), and plasma enhanced chemical vapor deposition. As described in U.S. Pat. No. 6,235,572 to Kunitomo et al., tantalum pentoxide films are generally deposited in an amorphous state. To improve the dielectric constant of the tantalum pentoxide, the films are subjected to a thermal treatment to give the film a crystalline structure. The crystalline structure of tantalum pentoxide films present a thin poly-crystal film having a grain boundary that is subject to current leakage between electrodes disposed on opposite sides thereof. Although increasing the film thickness may reduce the leakage current and increase the capacitance, too great an increase exacerbates leakage current due to the increased stress it places on the tantalum pentoxide film. To reduce current leakage while maintaining sufficient capacitance, Kunitomo et al. advocate forming a multi-layered tantalum pentoxide film employing CVD techniques.
U.S. Pat. No. 5,936,831 to Kola et al., recognizes that capacitors fabricated with anodized reactively sputtered Ta2O5 films were found to have satisfactory leakage and breakdown properties, but degraded upon thermal annealing above 200xc2x0 C. The degradation demonstrated irreversible increases in the temperature coefficient of capacitance (TCC), as well as the dissipation factor. These are believed to be caused by diffusion of electrode metal atoms into the dielectric and diffusion of oxygen out, creating oxygen deficiency defects. To overcome this degradation, Kola et al. discuss using a variety of metals for the electrodes, including aluminum (Al), chromium (Cr), copper (Cu), tantalum nitride (TaNx), titanium nitride (TiNx) and tungsten (W). As a result, Kola et al. advocate forming a thin-film capacitor with a dielectric formed from nitrogen or silicon-doped tantalum oxide and at least one electrode formed from chromium by anodically oxidizing TaN or Ta2Si and forming a Cr electrode.
U.S. Pat. No. 6,207,489 to Nam et al., discloses a method for manufacturing a capacitor having a dielectric film formed from tantalum oxide. The method includes forming a lower electrode that is electrically connected to an active region of a semiconductor substrate. A pre-treatment film including a component selected from a group consisting of silicon oxide, silicon nitride, and combinations thereof, is formed on the surface of the lower electrode. A dielectric film is formed on the pre-treatment film using a Ta precursor. The dielectric film includes a first dielectric layer deposited at a first temperature, which is selected from a designated temperature range. A second dielectric layer is deposited at a second temperature, which is different from the first temperature. A thermal treatment is thereafter performed on the dielectric film in an oxygen atmosphere.
There is a need, therefore, to provide a technique for producing thin film capacitors having sufficient capacitance and break down voltage, while minimizing current leakage.
A capacitor and a method of forming the same is disclosed, one embodiment of which includes depositing a multi-layer dielectric film between first and second spaced-apart electrodes. The multi-layer dielectric film includes first and second layers that have differing roughness. The layer of the dielectric film having the least amount of roughness is disposed adjacent to the first electrode. After depositing the second layer of the dielectric film adjacent to the first layer, the second layer is annealed. It is believed that the reduced roughness of the first layer reduces pin-hole formation in the second layer. In this manner, current leakage is prevented, or reduced, in the resulting thin-film capacitor, while providing suitable capacitance and breakdown voltage. An exemplary embodiment of the thin-film capacitor forms the dielectric material from silicon dioxide (SiO2) and tantalum pentoxide (Ta2O5). To provide the thin-film capacitor with the desired operational characteristics, the layer of tantalum pentoxide is provided with a thickness that is approximately three times greater than the thickness of the silicon dioxide layer. To that end, in the exemplary method, the first electrode is formed by vapor deposition of phosphorous oxytrichloride (POCl3). The layer of silicon dioxide is formed by thermal oxidation of silicon in oxygen. The layer of tantalum pentoxide is formed by sputtering tantalum (Ta) onto the layer of silicon dioxide in an oxygen rich ambient. Annealing then densities the tantalum oxide. Thereafter, the second electrode is formed by deposition of a layer consisting of aluminum, chromium, copper, titanium, titanium nitride, titungsten or a combination thereof.